Radome structures are conventionally used to protect equipment, such as microwave antennae, from the physical environment. It is also desirable to shield such equipment from external incident electromagnetic energy which can adversely affect the electrical operating characteristics thereof. Such a shield, during the operation of the equipment, e.g., an antenna system, should be transparent to the energy only in the selected frequency range handled by the antenna equipment and only when the equipment is placed into operation. When the equipment is not operating, such a shield should reject electromagnetic energy within such frequency range as well as outside such frequency range.
Radome shields having such characteristics are often referred to as "shutter-type" radomes, the shutter being effectively "closed" to all frequencies both within and outside the frequency band of interest during non-operation and the shutter being effectively "open" only to frequencies in the desired operating frequency operating portions of the spectrum during operation, e.g. when antenna equipment within the radome is operating.
Another shutter arrangement for providing what has sometimes been referred to as "complementary" shutter operation is disclosed in U.S. Patent application Ser. No. 642,536 now U.S. Pat. No. Des. 287,592, filed by Jean-Claude Sureau on Aug. 20, 1984, in which transmission of electromagnetic energy through the structure is permitted in the "open" shutter mode over a relatively wide frequency band which is generally established as being substantially wider than a particular frequency band of interest. During such mode the structure is essentially operating as a non-resonant structure. In the "closed" shutter mode, the structure is made essentially resonant at the center frequency of the desired selected frequency band so as to effectively suppress all transmission at such center frequency and to substantially reduce the energy in the remaining portion of the selected frequency band about the center frequency. Such a structure is said to operate as a suppression resonant structure as opposed to the above described transmission resonant shutter structure and, hence, the use of the term "complementary". Such a structure normally utilizes a symmetrical pattern of symmetrical conductive elements with diodes interconnecting adjacent conductive elements both in the horizontal and vertical directions. When the diodes are appropriately biased in a conductive direction (forward biased) the shutter operates in its open shutter mode and when the diodes are in their non-conductive state (reverse or zero biased) the shutter operates in a closed shutter mode.
However, is some applications requiring such a complementary shutter operation it may be desirable, and in some cases necessary, to provide shutter operation in the opposite sense, that is, to produce a closed complementary shutter operation when the diodes are forward biased and to produce an open complementary shutter operation when the diodes are reverse or zero biased. For example, in some environments when the equipment which is protected by the radome shutter structure is operating, there may not be power available for biasing the diodes in the appropriate manner and, accordingly, the operative condition needs to be arranged so that it can be effected without using power for diode biasing purposes.